In the future, robotics will rely heavily on control algorithms operating in the domain of force and torque control, since this permits applications which cannot be implemented with position control alone. For example, without an accurate measurement of the forces generated by a robot, it is impossible to handle with precision delicate non-modelled objects or interact with man in safe conditions in an unstructured environment. The rotary actuators used nowadays have considerable limitations which make them unsuitable for this type of control.
The characteristics which a rotary actuator should ideally possess are the following:                high maximum torque,        high maximum angular speed,        high power/weight ratio,        high energy performance,        high force generation bandwidth, and        low mechanical impedance in the event of position disturbances.        
The term “mechanical impedance” is to be understood as referring to the apparent inertia of a system in response to a disturbance, namely the torque effectively produced when a disturbance (force or torque) is applied to an actuator which is programmed so as to produce zero torque. The torque which is effectively produced depends on the speed at which the disturbance is applied, for which reason the mechanical impedance is often analysed with reference to a particular frequency. With fast control loops it is possible to reduce the mechanical impedance at speeds lower than the frequency of the control loop, but at higher speeds only mechanical effects are involved. In real systems, it is never possible, even below the control frequency, to eliminate completely the mechanical impedance owing to the frictional forces and the inertial forces due to the non-suspended mass.
Most currently available rotary actuators for robotic applications have a limited force/torque control capacity. Hydraulic actuators have excellent maximum torque, maximum speed and power/weight ratio characteristics, but very high mechanical impedance. Therefore it is difficult to use these actuators in actuating systems with torque control. Pneumatic actuators have an intrinsic compliance because they contain a compressible gas, but they are affected by band control problems owing to the limits of their valves in terms of flow rate and tightness. Electric actuators are fast, but generally they are able to produce low torques and therefore require the use of reduction units which are subject to high friction and therefore increase considerably mechanical impedance. One way to improve the torque control properties of electric actuators is to use an elastic element—typically a spring made of metallic material—for connecting the electric motor to the external load. If the position of the external load is perturbed by a disturbance, the force in the spring starts to increase. Since, however, the inertia of the spring is very low, this force varies very gradually, unlike that which happens with the use of reduction units which have high inertia. In this way, the addition of an elastic element in series decouples to a certain extent the movements of the electric motor and of the external load. Even though the addition of an elastic element reduces the capacity of the actuator to generate rapidly a high torque, since the elastic element must be deformed significantly in order to be able to generate a high force, it also increases the resistance to breakage in the event of pulsed loads and in particular the reliability of the force/torque control. Elastic rotary actuators are therefore becoming more and more widespread in the robotics sector.
An elastic rotary actuator of the type mentioned above is described in EP1972414 and comprises a cam mechanism acting on the elastic means so as to produce a deformation of said elastic means which is variable in a non-linear manner depending on the angle of rotation of the final output member. Such actuators, however, still suffer from significant performance problems which limit their use in certain applications.